1. Field of the Invention
The present invention relates to a binder for coating materials containing dispersed inorganic powder, a coating material containing dispersed inorganic powder, and a magnetic recording medium. More particularly, the present invention relates to a binder for coating materials containing dispersed inorganic powder and a coating material containing dispersed inorganic powder which are excellent in dispersion of inorganic powder and stability of the dispersion and provide a coating layer having advantageous properties, and a magnetic recording medium which uses the coating material as the magnetic coating material, and provides a magnetic tape and the like having excellent magnetic properties, durability, and running property.
2. Description of the Related Art
A coating material generally contains three main components which are a binder for forming a coating layer, a solvent, and a pigment or a magnetic powder. Auxiliary materials are added, if necessary. The binder for forming a coating layer is the component which is converted into a solid film after the coating material is dried. The binder is the most important component of a coating material. The solvent has an important role of adjusting viscosity of a coating material, helping dispersion of pigment in the coating material, and enabling smooth coating of a surface and formation of a film. The pigment includes coloring pigments which are used for the purpose of coloring a coating layer, fillers which are used for the purposes of improving workability in the coating operation and enhancing mechanical properties of the coating layer, and special functional pigments which provide the coating layer with special functions, such as prevention of corrosion and the like. A magnetic coating material contains magnetic powder dispersed in a mixture of a binder for forming a coating layer and a solvent in place of the pigment dispersed similarly. By applying a magnetic coating material to a non-magnetic substrate to form a magnetic layer, a magnetic recording medium such as a magnetic tape, a magnetic disk, and a magnetic card, can be prepared.
Recently, a higher recording density is required for magnetic recording media. For example, it is required that magnetic tapes have a higher saturated density of magnetic flux Bm, a higher rectangular ratio Rs (residual density of magnetic flux Br/saturated density of magnetic flux Bm), lower roughness of the surface, and, furthermore, better flexibility and abrasion resistance. As a higher S/N ratio is being required, it is required that fine magnetic powder having a higher specific surface area such as 30 m.sup.2 /g to 70 m.sup.2 /g be used, and a binder should have a combination of more excellent dispersing property and ability to form tougher binding so that the fine magnetic powder is homogeneously dispersed and firmly bound.
However, satisfactory results with respect to any of the dispersion property of the magnetic powder and the abrasion resistance of the magnetic layer cannot be obtained by using binders which have heretofore been used, such as polyester resins, cellulosic resins, polyurethane resins, phenol resins, epoxy resins, polyamide resins, and homopolymers and copolymers using vinyl chloride, vinylidene chloride, vinyl acetate, (meth)acrylic acid esters, (meth)acrylamide, (meth)acrylonitrile, butadiene, styrene, vinyl esters, and the like.
Among these binders, polyurethane resins have been widely used because of good abrasion resistance. However, polyurethane resins have a drawback in that it has poor ability to adsorb magnetic powder, and therefore, particularly poor ability to disperse fine magnetic powder having a large specific surface area described above.
For overcoming the above drawback in conventional polyurethane resins, incorporation of a residue group of a sulfonic acid salt or the like into the molecular chain has been proposed [Japanese Patent Publication Nos. Showa 58(1983)-41565 and Showa 63(1988)-55549]. However, when a polyurethane resin has a low molecular weight, the coating layer formed has low abrasion resistance and strength even though the dispersion property is improved. When the molecular weight is increased for improving abrasion resistance and strength which are the characteristics of the polyurethane resin, the effect of improving dispersion of magnetic powder cannot be obtained. Furthermore, when the molecular weight is increased, viscosity of the magnetic coating material containing dispersed magnetic powder is increased and dispersion of the magnetic powder deteriorates.
Because viscosity of a magnetic coating material tends to be increased when a finer magnetic powder is used, improvement of strength by methods other than the increase in the molecular weight is necessary. Crosslinking of a binder with a polyisocyanate compound is widely utilized as one of such methods. For enabling the crosslinking by a polyisocyanate compound, incorporation of a functional group having an active hydrogen which can react with the isocyanate group, such as hydroxyl group as a representative example, into the binder has been adopted.
However, for exhibiting sufficient strength with a polyurethane resin which has a decreased molecular weight for improving the dispersion property, a large amount of hydroxyl group must be incorporated into the molecule as the crosslinking points. When a large amount of hydroxyl group is incorporated, a magnetic coating material containing fatty acids or fatty acid esters which are generally used as lubricants shows decrease in the stability of dispersion, and a magnetic recording medium having good properties cannot be produced.
It is also proposed that epoxy group is incorporated into a polyurethane resin and used for crosslinking [Japanese Patent Publication No. Showa 55(1980)-40927, Japanese Patent Application Laid-Open No. Heisei 1(1989)-109523]. However, it has been found that the binder thus produced has too poor ability for dispersion and cannot be applied to fine magnetic powder.
For solving these problems, the present inventors have proposed that a magnetic recording medium having excellent durability can be obtained by combining a polyurethane resin having a polyfunctional isocyanate group with a vinyl chloride polymer in particular and using the combination as a binder [Japanese Patent Application Laid-Open No. Heisei 3(1991)-141020]. However, it is frequently practiced in recent years that metal magnetic powder having a strong adhesive force is dispersed by using an apparatus having a high shear force, such as a pressurized kneader. In such cases as the kneading under a condition of an extreme high viscosity, heat generation tends to take place. This situation occasionally causes another problem that dispersion deteriorates against the intention because reaction of the reactive binder further proceeds during the kneading process.